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The Ouachita thrust fault belt, a large, relatively untested hydrocarbon province, contains more than 30,000 ft (9,100 m) of Carboniferous flysch rich in potential source and reservoir rocks. To estimate the thermal maturity of these strata, vitrinite reflectance (in oil) was measured from more than 90 bulk samples of the Carboniferous-age Stanley, Jackfork, Johns Valley, and Atoka Formations. Inasmuch as no subsurface samples were available, the freshest possible outcrop samples were used for the analysis, despite the possible deleterious effects of oxidation on the accuracy of measured reflectance values.
Iso-reflectance contours generally trend parallel to structural grain in the western two-thirds of the Ouachitas. The "core" areas where pre-Carboniferous strata are exposed, as well as areas immediately adjacent to the core, are well defined by reflectance values greater than 2.0%. Outward from the core areas toward the north and south, reflectance values tend to decrease, although some minor variations owing to complex structure are present. In Arkansas, samples from the thrust-fault belts both north and south of the Benton uplift yield reflectance values between 1.0 and 2.0%. In Oklahoma, samples from the area north of the Broken Bow uplift yield reflectance values between 0.5 and 1.0%.
In the eastern third of the Ouachitas, iso-reflectance contours obliquely cut structural grain, and reflectance values are significantly higher. Samples from the Benton uplift give reflectance values higher than 3.0%, and measured values approach 5.0% from samples in and near the core. Although there is a general decrease outward from the core area to both the north and south, reflectance values greater than 2.0% characterize the entire width of the Ouachitas in this eastern area.
Reflectance values obtained from samples collected from both sides of major thrust faults in the western Ouachitas reveal that older, upthrown strata are more thermally mature than younger, downthrown strata. In contrast, samples collected from analogous structural positions in the eastern Ouachitas display identical thermal maturities on both upthrown and downthrown sides of thrust faults.
In the western two-thirds of the Ouachitas, stratigraphic depth of burial appears to have been the primary factor that controlled thermal maturity. The Carboniferous strata at the surface in this area are well within the window of oil and gas generation and preservation. The anomalously high thermal maturity of Carboniferous strata in the eastern third of the Ouachitas is probably the result of heat dissipated from Mesozoic rifting and intrusive events. This thermal overprint places the maturity of these strata beyond the limits of oil preservation and locally beyond the limits of wet gas preservation.
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